Displaced electrode amplifier

Abstract

A displaced electrode amplifier (“DEA”) for measuring signals from high impedance sources. The amplifier may include an operational amplifier (“op-amp”) configured as a unity gain buffer, with a feedback path to the non-inverting input to at least partly compensate for a parasitic input shunt impedance. In cases where the device is to measure AC signals in high ambient temperatures, the non-inverting input may be coupled via a large resistance to a ground reference that is driven with a second feedback signal to magnify the effective value of the large resistance. Where a differential configuration is desired, one or more tuning resistors may be provided to match responses of different input buffer stages, thereby maximizing the common mode rejection. The disclosed amplifier is suitable for use in oil-based mud resistivity imaging tools but is also suitable for other applications.

1. A displaced electrode amplifier that comprises:
 a first operational amplifier (“op-amp”) having an inverting input, a non-inverting input, and an output, wherein the output is coupled to the inverting input to configure the op-amp as a unity gain buffer; 
 a first feedback impedance coupled between the output and the non-inverting input to at least partly compensate for a parasitic shunt impedance coupled to the non-inverting input; 
 a second op-amp having an inverting input, a non-inverting input, and an output, wherein the output of the second op-amp is coupled to the inverting input of the second op-amp to configure the second op-amp as a unity gain buffer; 
 a second feedback impedance coupled between the output of the second op-amp and the non-inverting input of the second op-amp to at least partly compensate for a parasitic input shunt impedance; and 
 a differential amplifier stage coupled to the outputs of the first and second op-amps to produce an amplified difference signal. 
 
     
     
       2. The displaced electrode amplifier of  claim 1 , wherein the feedback impedance comprises a series combination of a capacitor and a resistor. 
     
     
       3. The displaced electrode amplifier of  claim 1 , further comprising:
 an input impedance coupled between the non-inverting input of the first op-amp and a sensing electrode. 
 
     
     
       4. The displaced electrode amplifier of  claim 3 , wherein the input impedance comprises a series combination of a capacitor and a resistor. 
     
     
       5. The displaced electrode amplifier of  claim 1 , further comprising:
 an electrically conductive shield for capacitance guarding the non-inverting input of the first op-amp to a sensing electrode; and 
 a resistor coupled between the output of the first op-amp and the electrically conductive shield. 
 
     
     
       6. The displaced electrode amplifier of  claim 1 , further comprising a large resistor coupled between the non-inverting input of the first op-amp and a reference node. 
     
     
       7. The displaced electrode amplifier of  claim 6 , wherein the reference node is driven by a third op-amp having its output coupled to its inverting input, and having its non-inverting input coupled to ground via a second resistor. 
     
     
       8. The displaced electrode amplifier of  claim 7 , wherein the non-inverting input of the third op-amp is further coupled to the output of the first op-amp via a capacitor to drive the reference node with positive feedback to magnify the effective value of the large resistor. 
     
     
       9. The displaced electrode amplifier of  claim 1 , wherein the differential amplifier stage includes a reference resistor coupled between the outputs of the first and second op-amps. 
     
     
       10. The displaced electrode amplifier of  claim 9 , wherein the differential amplifier further includes a tuning resistor coupled between a reference voltage and one of the outputs of the first and second op-amps, wherein the tuning resistor has a value selected to provide a null in a common mode rejection response of the displaced electrode amplifier. 
     
     
       11. An oil-based mud imaging tool that comprises:
 a sensor array having one or more voltage electrodes and one or more current electrodes, wherein the one or more current electrodes are energized by an excitation source to create an oscillatory current flow in a borehole wall; and 
 at least one displaced electrode amplifier coupled to the one or more voltage electrodes to measure a differential voltage created by the oscillatory current flow, wherein the displaced electrode amplifier employs positive feedback to nullify parasitic input shunt impedances of the voltage electrodes, wherein the displaced electrode amplifier comprises:
 two input buffer stages each coupled to a respective one of the voltage electrodes to provide the positive feedback, comprising:
 a first operational amplifier (“op-amp”) having an inverting input, a non-inverting input, and an output, wherein the output is coupled to the inverting input to configure the op-amp as a unity gain buffer; 
 a first feedback impedance coupled between the output and the non-inverting input to at least partly compensate for a parasitic shunt impedance coupled to the non-inverting input; 
 a second op-amp having an inverting input, a non-inverting input, and an output, wherein the output of the second op-amp is coupled to the inverting input of the second op-amp to configure the second op-amp as a unity gain buffer; 
 
 
 a second feedback impedance coupled between the output of the second op-amp and the non-inverting input of the second op-amp to at least partly compensate for a parasitic input shunt impedance; and
 a differential amplifier stage coupled to the input buffer stages to the outputs of the first and second op-amps to measure the differential voltage. 
 
 
     
     
       12. The tool of  claim 11 , wherein the differential amplifier stage also operates to match the responses of the input buffer stages in a frequency range of interest. 
     
     
       13. The tool of  claim 11 , wherein each of the input buffer stages includes a high impedance reference voltage that is not susceptible to bias currents that commonly develop under high operating temperatures. 
     
     
       14. The tool of  claim 11 , wherein each of the input buffer stages drives the high impedance ground reference with positive feedback to reduce loading in a frequency range of interest. 
     
     
       15. The tool of  claim 11 , wherein each of the input buffer stages further includes an electrically conductive shield for wiring to the corresponding voltage electrode, and wherein the electrically conductive shield is driven from an output signal of the input buffer stage. 
     
     
       16. A displaced electrode amplifier sensing method that comprises:
 buffering a voltage from a first input node with a first operational amplifier (“op-amp”) to produce a first output signal that is coupled to an inverting input of the first op-amp to configure the first op-amp as a unity gain buffer; 
 at least partly compensating for a parasitic shunt impedance with a first feedback impedance coupled between the first op-amp's output and a non-inverting input of the first op-amp; 
 buffering a voltage from a second input node with a second op-amp to produce a second output signal that is coupled to an inverting input of the second op-amp to configure the second op-amp as a unity gain buffer; 
 at least partly compensating for a parasitic shunt impedance with a second feedback impedance coupled between the second op-amp's output and non-inverting input of the second op-amp; and 
 combining the first output signal with the second output signal with a differential amplifier to produce an amplified difference signal. 
 
     
     
       17. The method of  claim 16 , further comprising:
 coupling the first input node to a high impedance reference node. 
 
     
     
       18. The method of  claim 17 , further comprising:
 driving the high impedance reference node with a feedback signal to increase an effective value of a resistor that couples the first input node to the high impedance reference node.